Embodiments of the invention relate generally to x-ray imaging devices and, more particularly, to an x-ray tube having an improved cathode structure.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The data acquisition system then reads the signals received in the detector, and the system then translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes typically include an anode structure for the purpose of distributing the heat generated at a focal spot. An x-ray tube cathode provides an electron beam from an emitter that is accelerated using a high voltage applied across a cathode-to-anode vacuum gap to produce x-rays upon impact with the anode. The area where the electron beam impacts the anode is often referred to as the focal spot. Typically, the cathode includes one or more cylindrically wound filaments positioned within a cup for emitting electrons as a beam to create a high-power large focal spot or a high-resolution small focal spot, as examples. Imaging applications may be designed that include selecting either a small or a large focal spot having a particular shape, depending on the application.
Conventional cylindrically wound filaments, however, emit electrons in a complex pattern that is highly dependent on the circumferential position from which they emit toward the anode. Due to the complex electron emission pattern from a cylindrical filament, focal spots resulting therefrom can have non-uniform profiles that are highly sensitive to the placement of the filament within the cup. As such, cylindrically wound filament-based cathodes are manufactured having their filament positioned with very tight tolerances in order to meet the exacting focal spot requirements in an x-ray tube.
In order to generate a more uniform profile of electrons toward the anode to obtain a more uniform focal spot, cathodes having a flat emitter surface have recently been developed. Typically a flat emitter may take the form of a D-shaped filament that is a wound filament having the flat of the “D” facing toward the anode. Such a design emits a more uniform pattern of electrons and emits far fewer electrons from the rounded surface of the filament that is facing away from the anode (that is, facing toward the cup). D-shaped filaments, however, are expensive to produce (they are typically formed about a D-shaped mandrel) and typically require, as well, very tight manufacturing tolerances and separately biased focus electrodes in order to meet focal spot requirements.
Thus, in another example of a flat surface for forming a filament, a flat surface emitter (or a ‘flat emitter’) may be positioned within the cathode cup with the flat surface positioned orthogonal to the anode. A flat emitter is typically formed with a very thin material having electrodes attached thereto, which can be significantly less costly to manufacture compared to conventionally wound (cylindrical or D-shaped) filaments and may have a relaxed placement tolerance when compared to a conventionally wound filament.
Despite being quite thin (perhaps a few hundred microns in thickness), however, electrons nevertheless tend to emit from the edge of the flat emitter, causing a non-uniform emission profile that can result in a non-uniform focal spot. As such, flat emitters typically include separately biased focus electrodes in order to meet focal spot requirements, as well.
A flat emitter typically includes support legs to provide both structural support to the flat emitter as well as a path for providing electrical current to the emitter. Thus, the emitter can rise significantly in temperature relative to the surrounding focusing structure (i.e., the cathode cup), which can lead to thermal growth of the support legs and to a change in position of the flat emitter relative to its surrounding cup. Such motion can cause a change in the focal spot position and shape during operation of the x-ray tube, leading to drift of the modulation transfer function (MTF) which can cause image artifacts to occur.
In WO/2009/013677, for instance, an electron emitter design as shown in FIG. 6 is presented that may reduce the negative influence of thermal growth of the emitter support legs. This emitter has an outer part that is mechanically connected to the inner part constituting the emission surface. During thermal growth the outer and inner part move together, thus reducing the negative influence on the focal spot.
However, the electron emitter design described in /2009/013677 still allows a relative displacement of the emitter with respect to the cathode cup during thermal growth of the emitter legs thus negatively influencing focal spot position and shape during operation of the x-ray tube.
Therefore, it would be desirable to have an apparatus and method capable of reducing or eliminating the effects of thermal growth of the legs of a flat emitter in an x-ray imaging device.